In recently disclosed “wireless sensor networks”, sensor nodes, including a wireless communication unit and a sensor, are provided on observation areas to collect sensing information in a “collection cycle (i.e., a sample cycle of a sensing value)”. For example, each sensor node in the wireless sensor network includes an energy harvesting unit that generates electricity by converting environmental energy (e.g., sunlight or oscillations) into electric energy; and a battery that stores electricity that is generated by the energy harvesting unit. Furthermore, each sensor node operates by using the electricity that is stored in the battery.
Furthermore, the sensing value, measured by each sensor node in the wireless sensor network, is reported to “a control device (i.e., a network device)” via a gateway device. Here, for example, the wireless sensor network has a tree-type topology. Specifically, the wireless sensor network includes a sensor node (i.e., “relay node”) that is in a “relay operating state” to relay transmission of the sensing value that is measured by a different sensor node; and a sensor node (i.e., “end node”) that is in a “non-relay operating state”. The sensing value, measured by each sensor node, is transmitted from each sensor node to the gateway device directly or via one or more relay nodes. That is, the wireless sensor network includes “the multihop communication path”.
Furthermore, the relay node transmits the sensing value of its own and transfers (i.e., receives and transmits) the sensing value of the end node that is located under it; therefore, compared to the end node, “the power consumption speed (the amount of consumed power)” in one collection cycle is high. Therefore, there is a possibility that “the amount of stored power” of the battery of the relay node becomes insufficient and the relay node has difficulty in operating in a stable manner. That is, there is a possibility that the multihop communication path becomes unstable.
Therefore, there is a disclosed technology in which the amount of stored power of each sensor node in the future is predicted in accordance with the prepared “prediction model (target function)” and the multihop communication path is selected based on the predicted amount of stored power.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-211389
However, the electrical characteristics of batteries (secondary batteries, such as lithium batteries) vary widely depending on batteries (i.e., individuals). Specifically, the battery electrical characteristics are such that the electromotive forces may be considered to be constant until certain timing but, after the certain timing, the electromotive forces are rapidly reduced (dropped). Furthermore, there are variations in batteries as to the timing of dropping, the way of dropping, etc. Moreover, the battery electric characteristics are changed due to effects of the external environment, such as temperature. Hence, it is difficult to prepare the prediction model itself.
Furthermore, it is considered that, in order to stabilize the multihop communication path, before the amount of stored power of the relay node reaches “a dangerous level”, in which it is difficult for the relay node to operate in a stable manner, the state of the relay node is changed into “a non-relay operating state”, that is, the relay node is changed into an end node.
However, as described above, there are variations in batteries as to the timing of dropping, the way of dropping, etc. Furthermore, the battery electric characteristics are changed due to effects of the external environment, such as temperature. Therefore, it is difficult to previously set the above-described “dangerous level”.